Three-way conversion (TWC) catalysts have utility in a number of fields including the treatment of exhaust gas streams from internal combustion engines, such as automobile, truck and other gasoline-fueled engines. Emission standards for unburned hydrocarbons, carbon monoxide and nitrogen oxide contaminants have been set by various governments and must be met by older as well as new vehicles. In order to meet such standards, catalytic converters containing a TWC catalyst are located in the exhaust gas line of internal combustion engines. Such catalysts promote the oxidation by oxygen in the exhaust gas stream of unburned hydrocarbons and carbon monoxide as well as the reduction of nitrogen oxides to nitrogen.
Known TWC catalysts which exhibit good activity and long life comprise one or more platinum group metals (e.g., platinum, palladium, rhodium, rhenium, and iridium) disposed on a high surface area, refractory metal oxide support, e.g., a high surface area alumina coating. The support is carried on a suitable carrier or substrate such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure, or refractory particles such as spheres or short, extruded segments of a suitable refractory material. TWC catalysts can be manufactured in many ways. U.S. Pat. No. 6,478,874, for example, sets forth a system for catalytic coating of a substrate. Details of a TWC catalyst are found in, for example, U.S. Pat. Nos. 4,714,694 and 4,923,842. Furthermore, U.S. Pat. Nos. 5,057,483, 5,597,771, 7,022,646, and WO 95/35152 disclose TWC catalysts having two layers with precious metals. U.S. Pat. No. 6,764,665 discloses a TWC catalyst having three layers, two of which have precious metals.
WO 2008/024708 discloses a layered, three-way conversion catalyst having the capability of simultaneously catalyzing the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides. In one or more embodiments, the catalyst comprises three layers in conjunction with a carrier: a first layer deposited on the carrier and comprising palladium deposited on a refractory metal oxide and an oxygen storage component; a second layer deposited on the first layer and comprising rhodium deposited on a refractory metal oxide and an oxygen storage component; and a third layer deposited on the second layer and comprising palladium deposited on a refractory metal oxide.
WO 2008/097702 relates to a multilayered, three-way conversion catalyst having the capability of simultaneously catalyzing the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides. Disclosed is a catalytic material of at least four layers in conjunction with a carrier, where each of the layers includes a support, at least three layers comprise a precious metal component, and at least one layer comprises an oxygen storage component (OSC). The catalytic material can further comprise a fifth layer, where at least four layers comprise a precious metal component, at least one layer comprises an oxygen storage component, and at least one layer is substantially free of an oxygen storage component.
Multilayered catalysts are widely used in TWO. Generally, vehicles require catalysts having the same general overall conversion functionalities, but different vehicle platforms dictate the configurations on the catalyst of individual functions. For example, the engine control of a particular vehicle dictates whether, for example, HC or NOx conversion will be the determining factor to meet regulation targets. These critical factors lead to catalysts designed with different outer-most layer favoring either HC or NOx conversion. As such, there is need to provide TWC catalysts that meet market needs, without introducing complexities into the manufacturing process. There is also a goal to utilize components of TWO catalysts, especially the precious metals, as efficiently as possible.
Multilayered catalysts are formed by deposition of washcoats onto the carriers or substrates. In some manufacturing processes, deposition of washcoats along a length of the carrier or substrate is limited. For example, sometimes a single pass of a washcoat covers less than 100% of the length of the catalyst, for example, only about 80-90%.
It is a continuing goal to develop three-way conversion catalyst systems that have the ability to oxidize hydrocarbons and carbon monoxide while reducing nitrogen oxides to nitrogen.